1. Field of the Invention
The present invention relates generally to the field of fluid dynamics and heat transfer, and more specifically to a system and method for mixing fluid streams within an industrial drying machine.
2. Description of the Prior Art
Industrial machines, such as those common in the textile, nonwovens and paper manufacturing industries, commonly utilize heated air to dry a newly formed product, as well for thermal bonding, curing and other processes that require an air stream with a uniform temperature profile. Typically, air is heated through conventional combustion means and then directed in various fashions towards the web of wet material. The heated air passes through or impinges the web, losing some of its heat in the drying process. The cooled air, referred to as system air, is then divided into portions that are re-circulated through the drying machine and portions that are exhausted into the atmosphere.
Drying machines in the aforementioned industries are generally of three types: through-air-dryers (TAD), impingement dryers, or floatation dryers. Each of these types of dryers is typically contained within a drying hood, which supplies and directs heated air to the surface of the web. A vacuum or pressure differential pulls the heated air through or onto the surface of the web and exhausts the cooled air into the system at large, at which point a portion of the cooled air will be exhausted into the atmosphere while the remainder is reused for drying applications. The direction of travel of the web is referred to as the machine direction, and the direction perpendicular to the machine direction and coplanar with the web is referred to as the cross-machine direction.
A typical dryer system 100 is shown in FIG. 1. As noted, the system 100 includes a dryer 110 that is partially surrounded by a dryer hood 112, through which air is drawn from the surrounding structures. A web of goods enters the hood 110 on the wet end 114 and proceeds through the dryer 110, where heated air is drawn through it, to the dry end 116. The heated air is pushed in through an intake 118 and is drawn out of an exhaust 120 by a main fan 122 which drives partially closed circuit as shown in FIG. 1. A portion of the system air is exhausted into the atmosphere through duct 124.
The remaining system air is directed to an air heater 126 that combines the system air with combustion products from a burner 128. The burner 128 is driven by a combustion air source 130, such as a fan, and fuel 132. The mixed air 134 is a combination of combustion products and system air that will be used to dry the web passing through the dryer hood 112. Those skilled in the art will recognize that the combination of the system air and the combustion products will not necessarily produce a uniformly profiled stream of heated air. On the contrary, the introduction of a secondary stream of combustion products into the system air may produce non-homogenous profile for the mixed air 134. As a result, a typical dryer system 100 generally incorporates a static mixer 136 for inducing turbulence and mixing into the mixed air 134 stream so as to maximize thermal uniformity prior to entering the drying hood.
The foregoing example demonstrates both the strengths and weaknesses of the state of the art heating systems. While the current art is able to make remarkable use of system air through the re-circulation mechanisms, the necessary mixing of that air with combustion products is potentially hazardous to the end product. An essential aspect of textile and paper manufacturing is that the air that is drawn through or impinged upon the product must have a substantially uniform temperature profile along the cross-machine direction. Particularly for the manufacture of lightweight materials, such as tissue paper, any deviation in the temperature profile can irreversibly damage the finished product. The economic effects of non-uniform heating are multiple, including the energy required to replace the lost product, the costs of replacing the wasted raw materials, and the labor necessary to fix, maintain, manage and operate the dryer through a new production cycle. As such, one of the paramount concerns in the paper industry is designing a dryer that reliably maintains a uniform temperature profile in the cross-machine direction.
As noted above, it is common practice to re-circulate spent system air and reuse it in the drying cycle. Typically, the system air is combined with newly heated air and then the air is mixed as it passes through the machine ductwork towards the web of goods. Although the industry has made several attempts at efficiently re-circulating the air exhausted through the roll, the current state of the art requires a significant distance between the mixing point and the web in order to ensure that the temperature profile of the mixed stream is sufficiently homogenous.
For example, attempts have been made to introduce a heated fluid stream into a cooler fluid stream by using a baffling structure. Such a mechanism was contemplated in the invention described in international publication WO/0012202 published on Mar. 9, 2000. Although that invention describes a mechanical means for inducing turbulence, and hence mixing, in the combination of two fluid streams, it still does not do so with optimal efficiency of space and energy. In particular, the baffle design does create a large eddy that induces mixing of the fluid streams, but it does not do so in a symmetrical or uniform manner. Thus, the designers must either remix the turbulent air with a second device such as a static mixer; or alternatively, they must maximize the distance between the baffle location and the intake into the drying hood. Each of these two solutions involves non-trivial modifications to the drying systems described above, and both solutions would cost the producer in terms of energy efficiency and space utilization.
Given the foregoing, it is readily apparent to those skilled in the art that there is a need for a system and method for mixing fluid streams that is compact, energy efficient and produces a reliably uniform temperature profile across the web. Moreover, there is a need in the art for solutions that can be easily integrated into current drying system design without greatly expanding the hardware and space necessary to manufacture textiles. Lastly, there is a need in the art for a drying system that will minimize energy expenditures while deriving the greatest benefits from the raw materials processed therein.